1. Field of the Invention
The present invention relates to a semiconductor device including an integrated circuit with an insulated gate field effect transistor having an active layer made of single crystalline semiconductor, and a method of fabricating the same. Particularly, the present invention is suitable for use in an electro-optical device typified by a liquid crystal display device or electroluminescence (EL) display device in which a pixel portion (active matrix circuit) and a driver circuit connected to the pixel portion are provided on the same substrate, and an electronic equipment incorporating the electro-optical device. Incidentally, in the present specification, the term “semiconductor device” indicates any devices functioning by using semiconductor characteristics, and includes the foregoing electro-optical device and the electronic equipment incorporating the electro-optical device in its category.
2. Description of the Related Art
In a flat plate type display device (flat panel display) typified by a liquid crystal display device, an EL display device, or the like, there is known a technique for fabricating an active matrix type display device by using an insulated gate field effect transistor (hereinafter, a field effect transistor is referred to as an “FET”) formed on a single crystalline semiconductor substrate. This is different from the case where a thin film transistor (hereinafter referred to as a “TFT”) is formed on a glass substrate or quartz substrate and an active matrix type display device is fabricated, and it is possible to directly apply the technique developed in a large scale integrated circuit (LSI), and there has been a merit in that high performance FETs operating at high speed and driven by low voltage can be formed and integrated on a substrate to a high density. However, on the other hand, it has been considered that the technique has such defects that this is restricted to a reflection type or spontaneous light emitting type display device since the substrate is opaque to visible light, or a single crystalline semiconductor substrate is limited to a size supplied on the market.
However, in the technical direction, such as improvement of picture quality or full digitization, in a field of a display device, performance improvement required for the active matrix type display device has inevitably become high. In the active matrix type display device, such structure has been adopted that several tens to several million transistors (TFT, FET, etc.) are arranged in a pixel portion for carrying out picture display, and a pixel electrode is connected to each of the transistors. Then, the device is constructed such that a voltage applied to each pixel is controlled by a switching function of the transistor to drive a liquid crystal or to cause an EL component to emit light so that a picture is displayed. For example, in the case of a liquid crystal display device, an opposite electrode is provided at the side of an opposite substrate with an intervening liquid crystal, and a kind of capacitance (in the present specification, it is referred to as “liquid crystal capacitance”) with the liquid crystal as a dielectric is formed. By controlling the electric charge stored in the liquid crystal capacitance, the liquid crystal is driven, and the amount of light transmitted through the liquid crystal is controlled to display a picture. However, since the capacity of this liquid crystal capacitance is gradually decreased by a leak current, the amount of transmitted light is changed and deterioration in the contrast of picture display has been caused. Then, it has been necessary to provide another capacitor (hereinafter referred to as a “storage capacitance”) different from the liquid crystal capacitance for each pixel. This storage capacitance functions to compensate capacity which the liquid crystal capacitance has lost, and is provided to hold the electric charge in a period of one frame from writing to next writing. In an EL display device, such construction has been adopted that when a switching transistor provided for each pixel is turned on, a current flows to a current controlling transistor by a signal corresponding to picture data, and the EL component spontaneously emits light.
In such an active matrix type display device, it is expected that a digital system becomes the mainstream in order to realize further improvement in fineness and improvement in density of picture information in the future. For that purpose, it is necessary to newly provide a circuit necessary for processing digital signals, such as a Digital/Analog (D/A) converter or a latch circuit, and the structure of a driver circuit becomes complicated as compared with a conventional analog system, and it has been a problem to form a display device by a driver circuit with a transistor capable of operating at high speed. However, in a TFT formed on a glass substrate or quartz substrate, even if polycrystalline silicon is used for its active layer, an electron mobility to be obtainable is about 400 cm2/V·sec, which is about ⅓ of that of an FET fabricated by single crystalline silicon.
Further, when a pixel density is increased, an area of a pixel electrode per pixel becomes small, and the ratio of occupation by wirings and transistors inevitably becomes large, so that there has been a problem in that an aperture ratio is lowered. Besides, since an area where a storage capacitance can be formed becomes small as the area of the pixel electrode is reduced, there has been a problem in that it becomes difficult to form, with a limited area, a capacitance necessary for driving a pixel portion.